Check out the video, skip to 45:26 for Linux part:
Below are the excerpts from the video:
"Valve announcing Steam Linux support changes things a bit but we have made two forays into Linux commercial market, most recently with Quake Live client but that platform just hasn't carried its weight compared to Mac."
"Its great that people are enthusiastic about Linux as a gaming platform but there are not many people who are interested in paying for a game and that seems to be the reality."
"One things that speaks in favor of Linux and open source is that Integrated graphics part is getting better and better. Intel has been completely supportive of open source graphic driver efforts."
"If I had time, I would love to work on optimizing Intel open source drivers."
"It is an enticing thought that there is a well supported open platform that you can deliver content through the Steam ecosystem, but its a tough sell on. But Valve gets a huge kudos for the vision they have."
If you don't know, John Carmack is a well-known advocate of open source software and he has publicly released the source code to Quake, Quake 2, Quake 3 and most recently Doom 3.